Several types of IC (integrated circuits), MEMS (micro-electro-mechanical systems) and optoelectronic/photonic devices need to be packaged in an inert or vacuum atmosphere to operate correctly. Outgassing, microleaks and diffusion are the most common mechanisms leading to a progressive increase of the pressure within vacuum/hermetic packages. These mechanisms also change the gas composition in the microcavity of hermetic packages. For instance, devices made from III-V semiconductor materials are found to be particularly sensitive to outgassed gas species such as hydrogen, while moisture affects many semiconductor and MEMS devices. Furthermore, changes in the ambient pressure in a microcavity can impair the operation of the devices housed therein. Likewise, the presence of undesired gaseous species can degrade the device performance even if their partial pressures remain very low. With the current trend in reducing the packaging size of IC's, MEMS, and optoelectronic/photonic microdevices, the control of the pressure, gas composition and gas concentration in the microcavities becomes a challenge, with the consequent increase in packaging costs and lower manufacturing yields. Long outgassing runs performed at high temperature are normally required to reduce future outgassing from the various package components once the sealing step has been completed, thus reducing the throughput of the packaging process along with increasing costs.
Examples of devices requiring a vacuum atmosphere maintained throughout their lifetime include, but are not limited to, IR (infrared) detectors (as shown in U.S. Pat. No. 5,895,233 to Higashi et al.), gyroscopes, microaccelerometers (as shown in U.S. Pat. No. 5,656,778 to Roszhart and U.S. Pat. No. 5,952,572 to Yamashita et al.), micromirrors, miniaturized resonators, solar cells, flat panel displays (as shown in U.S. Pat. No. 5,934,964 to Carella et al.), pressure transducer devices, and ring laser gyroscopes. Some other types of devices must be surrounded by an inert atmosphere maintained at a predetermined base pressure. The presence of moisture results in stiction problems in MEMS devices and to failures in semiconductor IC devices.
Gas-absorbing materials, commonly known as getters, are currently used to control the pressure inside sealed microcavities. As vacuum/hermetic packages shrink, the electrically-activated getters, well known in the art, get too bulky to fit in these microcavities. Using getter materials in the form of thin films can solve this problem.
In conventional getter systems, the thin-film getter material is deposited on structures having various shapes and it is heated either by bake out, by an electrical resistor, by RF (radio-frequency) heating or by absorption of laser radiation. Heating allows the getter material to reach its activation temperature, which is typically in the range of 400° C. to 1000° C. Such elevated temperatures permit the diffusion into the bulk of the getter material of the passivation layer that grows on the surface of the getter, thus leaving a fresh surface area ready for further capture of different gas species. The activation temperature depends on the composition of the getter material. For example, alloys of zirconium have an activation temperature lying between 300° C. and 1000° C., while getter films made up of elemental zirconium or of titanium must be heated to 600-700° C. Unfortunately, these high activation temperatures can damage the semiconductor devices and MEMS microstructures integrated in the packages. Furthermore, such high activation temperatures can damage the structural parts of a vacuum/hermetic package such as the hermetic joints, the antireflection coatings, and the die-attached materials. In addition, heating the whole assembly to the activation temperature of the getter can generate a great deal of thermal stress to the assembly. The stress comes from the close contact of components made up of materials having slightly different thermal expansion coefficients. It is also well known that subjecting the whole package to such elevated temperatures can induce outgassing. Solutions proposed in the prior art include placing the getter and the device into two separate compartments formed within the package. Other solutions rely on localized heating methods such as laser heating or electrical Joule heating. These solutions however, impact negatively on the complexity of the packages and on their costs.
INO (Quebec City, Canada), which is the current assignee of the present application, has developed a low-temperature vacuum micropackaging wafer-level or chip-to-wafer process for its VOx (vanadium mixed oxide) based microbolometer detectors. Further details can be found in S. Garcia-Blanco et al. (2008) “Low-temperature vacuum hermetic wafer-level package for uncooled microbolometer FPAs” (Proceedings of the SPIE vol. 6884, paper 68840P). This micropackaging process subjects the microbolometer chip to a temperature that does not exceed 140° C. In principle, this maximum temperature could be reduced further provided that an efficient getter is integrated in the package to reduce the temperature required for the outgassing step prior to vacuum sealing. For IR VOx-based microbolometer detectors, a very good vacuum (typically less than 10 mTorr for most applications) must be maintained over the lifetime of the package, which is typically 5 to 10 years. The lifetime of the package, defined here as the time it takes for the pressure inside of the microcavity to reach a certain maximum permissible value, depends on the amount of gases outgassed in the microcavity as well as on the volume of the microcavity. The micropackaging technology referred to above provides a ceramic spacer having an adjustable thickness in order to provide different cavity volumes.
Biocompatible packages and vacuum packaging of biocompatible devices are other examples wherein the heating of the package at elevated temperature is not an option. These devices rely frequently on functionalized molecules that cannot withstand elevated temperatures.
Another technique for achieving low-temperature vacuum-hermetic packaging is reported in M. R. Howlader (2010) “MEMS/microfluidics packaging without heating” (Proceedings of the SPIE vol. 7592, paper 75920H). To achieve a good vacuum, an outgassing run at an elevated temperature should be carried out for a prolonged period together with the integration of a performing getter, which is usually activated at temperatures ranging from 300° C. to 1000° C. Therefore, a device that would allow activation of the getter without undue heating of the neighboring components in the package would be a great advantage.
The getter material must be present in sufficient quantity in the package to ensure that the ambient pressure remains below a predetermined maximum value during the entire lifetime of the packaged device. The maximum permissible pressure depends on the particular application, but its value ranges from less than 10 mTorr to a few hundreds Torrs. The shrinking of the packages calls for the use of more and more compact getters. The cavity volumes of the packages of current MEMS, semiconductor, electronics or photonics devices vary from a few nanoliters to thousands of microliters, therefore limiting seriously the room space available for insertion of conventional getters, as described above.
As shown in the schematic sectional view of FIG. 1 (PRIOR ART), patents of the prior art such as U.S. Pat. No. 6,897,551 to Amiotti disclose the deposition of a non-evaporable getter (NEG) material 13 on an inner surface of the cover 12 of a package that defines a microcavity 14 in which a MEMS microdevice 11 is enclosed. This approach works provided that the cover 12 is not used as an optical window, thus precluding its implementation with MEMS microdevices that must interact with the outside via the capture and/or emission of optical radiation. Alternatively, the gettering material can be deposited in close proximity to the microdevice structure, so that there is no obstruction of any possible optical radiation path (see U.S. Pat. No. 6,534,850 to Liebeskind). Referring now to the cross-sectional side view of FIG. 2A (PRIOR ART), a thin-film non-evaporable getter 180 made up of a material 181 is deposited on the same substrate 140 as the electronic devices (e.g. transistors) 120 and the vacuum device 130, all of these devices requiring to be packaged inside a vacuum microcavity 194. During electrical activation of the thin-film getter 180, temperature sensors trig an alarm signal as soon as the temperature of the substrate 140 approaches a level that could damage the electronic devices 120. In the exemplary configuration shown in FIG. 2A, the thin-film getter 180 thermally connects to the substrate 140 through the dielectric layer 150. The layer 150 provides thermal insulation to protect the electronic devices 120, but this insulation may not be sufficient during activation of the getter 180. When the getter 180 reaches its activation temperature, the temperature rise of the substrate 140 and the heat conduction therein then call for safely spacing the electronic devices 120 from the thin-film getter 180. As a consequence, attempts at reducing further the whole package size are seriously compromised. In addition, the substrate 140 acts as a heat sink that raises the electrical power required to set the thin-film getter 180 at its activation temperature. The flowchart diagram of FIG. 2B (PRIOR ART) illustrates the way the thin-film getter 180 is activated.
Referring now to FIG. 3 (PRIOR ART), there is shown an exemplary configuration disclosed in U.S. Pat. No. 7,696,622 to Takemoto et al. in which continuous thin-film getters 40a, 40b, and 40c are deposited on the bottom surfaces and on the sidewalls of the vacuum microcavities 34. The getters serve to control the pressure inside the microcavities 34. Unfortunately, the getters 40a, 40b, and 40c are in close thermal contact with a lower glass substrate 32, thus raising the electrical power needed for their activation.
As depicted in the cross-sectional side view of FIG. 4 (PRIOR ART), U.S. Pat. No. 7,309,865 to Ikushima et al. teaches an approach for decreasing the electrical power required for proper activation of a getter film. The non-evaporable thin-film getter 185 is attached to the sidewalls 26 of a microcavity by means of microfabrication techniques. The conductance of the thermal path between the getter film 185 and the substrate 160 is therefore lowered, so that less electrical power is required to activate the getter. However, a specific microfabrication process flow is necessary to implement the configuration depicted in FIG. 4, the process flow being not compatible with that of the MEMS devices to be integrated in the microcavity. Furthermore, the surface area of the getter 185 is limited to the horizontal extent of the vacuum microcavity. Complex packaging strategies must be developed to implement the gettering scheme depicted in this figure.
Techniques for increasing the surface area of getters have been disclosed in U.S. Pat. No. 7,115,436 to Lutz et al., U.S. Pat. No. 5,701,008 to Ray et al., and U.S. Pat. No. 6,806,557 to Ding. In many cases, the getter is activated during the sealing of the lid (cover). This step can be performed by silicon direct bonding, anodic bonding, glass frit bonding, soldering, or eutectic bonding. The interested reader can refer for example to U.S. Pat. No. 6,499,354 to Najafi et al. and to D. R. Sparks et al. (2003) “Chip-level vacuum packaging of micromachines using nanogetters”, (IEEE Transactions on Advanced Packaging vol. 26, pp. 277-282). Unfortunately, the elevated temperatures (typically above 300° C.) required during performance of these bonding techniques can damage the devices (such as MEMS, photonic devices, or electronic devices) being packaged. Safer techniques for activating getters have been developed. These techniques include separate heating of the lid prior to final assembly and sealing, local laser heating (see U.S. Pat. No. 6,139,390 to Pothoven et al.) and getter activation by electrical pulses (see U.S. Pat. No. 6,534,850 to Liebeskind). Those skilled in the art will appreciate the increased complexity of these packaging processes and their subsequent impacts on the manufacturing costs and throughput.
From the above discussion and in view of the various solutions disclosed in the documents of the prior art reviewed above, there is a need for devising selective heating schemes to permit the activation of thin-film getters integrated inside vacuum/hermetic packages. These schemes must be devised in such a way that the heat generated during the process does not flow significantly towards the other components integrated in the microcavity. In addition, it is desired that these schemes be fully compatible with the process flows of current use for manufacturing MEMS devices to be packaged in order to keep the end costs of these products as low as possible.